Filter cartridges have a cylindrical filter element for filtering particulates from fluid and gas streams. Filters subject to radially inward (outside-to-inside) fluid flow typically have an interior core and an exterior cage. Filters subject to radially outward (inside-to-outside) fluid flow may not require an interior core. Some filters use a helical wrap rather than a cage to secure the filter element in the filter and reduce weight and cost.
Cageless, coreless filters subject to radially outward flow use single layer helical wraps that are very high in strength and modulus to resist differential pressures across the filter. The reinforced materials used in wraps are typically uniaxial; that is, they have high strength in the axial direction but little to zero strength in the cross-width direction. Use of high modulus fibers in wraps requires the use of materials with high fiber content by volume to be effective. As a result, wraps are difficult to work with due to fiber migration and handling. In addition, thermoplastic materials with high fiber content and high chemical compatibility do not bond well to hot adhesives commonly used in manufacturing filter elements.
Further, highly reinforced wraps used in many applications are very stiff and may lead to numerous undesirable manufacturing issues, including poor handling and performance, and low manufacturing yield. The stiff materials are difficult to manipulate while forming the filter element. The stiff material also has a tendency to separate from the ends of the filter element when cutting the filter element, which is undesirable during the end capping process. To end cap properly, the tails of the wrap may be trimmed. However, trimming the ends reduces the effective strength of the wrap and may lead to failures at pressures lower than designed. Because the highly reinforced wraps have little to zero strength in the cross direction, trimming the wrap prevents that load from being distributed along the entire width of the material. Only the width of the wrap bound in the end caps carries the load.